All Djinn
The only way to get all 28 Djinn from the original Golden Sun is to use a password or to transfer your file from that game to Golden Sun 2. All 72 Djinn from Golden Sun and Golden Sun 2 are used to open a sanctuary in the town near the Jupiter Lighthouse, where you can find a Boss called Dullahan who guards the ultimate summon in the game.

Revisit towns
Once you get the Teleport Psyenergy, you can use it on the world map to go to any town previously visited.

Sol Blade
The Sol Blade can be found at the Mars Lighthouse. It unleashes often and is the most powerful weapon in the game. Only Isaac or Felix can use this sword.

Darkside Sword
The Darkside Sword can be obtained by taking a Dark Matter to the blacksmith. It is rare to get from him, and is cursed.